An Analysis of Sexual Isolation in the Domestic Fowl: Ii. the Basis of Homogamy in Females

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(I) An experimental analysis of the mechanisms underlying homogamy in the domestic hen was made, because of the role of this trait in effecting potential sexual isolation between breeds and strains of chicken. (2) The principal stimuli and sensory modalities involved in breed and strain 'identification' were analysed by means of modification and comparative techniques. (3) It was demonstrated that the entire dimorphic plumage colour pattern of the Brown Leghorn cock is an important stimulus in 'identification' by females of that breed. However, females also discriminate between identically-coloured strains, implicating the involvement of other cues in the identification process. (4) Breed and strain specificity in male body size, courtship vocalisations, and non-vocal courtship patterns may have some isolating value, but is not of paramount importance in breed and strain 'identification' by hens. Specificity in other behavioural aspects of the phenotype (e.g. posture, deportment, activity level) was not examined, but should not be overlooked. (5) Homogamy appears to have a partly genetic basis in the Brown Leghorn hen, but specific (i.e. own-strain, heterosexual) prior experience is a necessary pre-requisite to the significant expression of the tendency under present conditions. (6) Heterosexual experience with strange-breed males enhanced sexual responsiveness to such and own-strain males; own-strain, heterosexual experience probably additionally enhanced stimulus specificity. (7) Wliilst no evidence suggesting that any brief, sharply-delineated, sensitive period for homogamy-enhancing experience existed in present material was forthcoming, it was demonstrated (a) that heterosexual experience prior to eleven weeks of age was not sufficient per se to effect a significant expression of homogamy subsequently and (b) that own-strain, heterosexual experience acquired after sexual maturity was efficacious as an enhancer of homogamy. (8) Differences in levels of solicitation behaviour and degree of homogamy exhibited between female groups deprived of own-strain, heterosexual experience for varying lengths of the juvenile phase are tentatively explained in terms of physical maturation and the ontogeny of social and sexual behaviour.